Hanging file folder support frames having support rails, corner fittings supported on legs and transverse members secured to the corner fittings are known to the art. Such a support frame having a generally C-shaped opening for the side entry of a rail is shown in U.S. Pat. No. 4,030,610. It is also known in the prior art to cantilever the top of the C-shaped corner fitting from the front portion of a rear wall which is thicker than the top wall. In accordance with the invention, a superior arrangement for controlling the flexing of the top wall to permit entry of the end of a side rail with facility and yet prevent its inadvertent dislodgment from the opening is provided.